1. Field of the Invention
This invention relates to sliding bearings of copper-based alloy. More particularly, the invention relates to a sliding bearing of copper-based alloy comprising a bearing alloy layer made of a copper alloy and a tin- or indium-containing overlay provided above the bearing alloy layer.
2. Description of the Related Art
There is known a sliding bearing of copper-based alloy which comprises a back metal layer, a bearing alloy layer made of a copper-based alloy and an overlay bonded onto the bearing alloy layer. The reason for disposing the overlay on the bearing alloy layer is that it enhances both the conformability and foreign matter embeddability of the bearing, which overlay is known to be made of a Pb-based alloy containing Sn and/or In or a Sn-based alloy.
In such sliding bearing of copper-base alloy, in the case where the overlay containing Sn and/or In is in direct contact with the bearing alloy layer, a brittle Cu--Sn compound layer occurs at a boundary defined between the overlay and the bearing alloy layer, which compound layer causes deteriorated fatigue resistance.
As a consequence, in the sliding bearing of copper-based alloy provided with the Sn-containing overlay, an intermediate layer of Ni or Ag is interposed between the overlay and the bearing alloy layer so as to prevent the brittle Cu--Sn compound layer from occurring.
With regard to the sliding bearing of copper-based alloy in which an intermediate Ni layer is laid, when the intermediate layer is exposed due to the wearing-out of the overlay, the corresponding shaft comes in direct contact with the intermediate layer hard in hardness, which is apt to cause seizure. In order to avoid the seizure, the intermediate layer has been formed to have a small thickness of 2 to 3 microns so that, even when it is exposed partially, such layer gets worn away and lost sooner so as to make the underlying layer or bearing alloy layer be exposed. In this way, improved seizure resistance has been attained by preventing the intermediate layer from being exposed at a time over a wide region of the surface of the bearing.
Another kind of bearing is known in which a multiplicity of grooves of several microns in depth are juxtaposed circumferentially of the bearing. This known construction is so designed that even upon wearing-down of an overlay, an intermediate layer cannot be exposed at a time over a large extent.
In Japanese Unexamined Patent Publication No. 7-190064 by the same assignee as in the present invention, still another bearing is disclosed which has an intermediate copper-zinc layer disposed between a bearing alloy layer and a lead-based overlay containing Sn and/or In. In the prior invention, Sn or In in the overlay is prevented from diffusing into the bearing alloy layer, consequently preventing a brittle Cu--Sn compound layer from occurring and hence improving seizure resistance through the action of the soft intermediate copper-zinc layer.
However, in the case of the provision of the grooves of several microns in depth or of making the Ni intermediate layer thin, it is difficult to further improve the seizure resistance. In addition, recently, a Pb-free or Pb content-reduced bearing has been desired in view of environmental influence of the disposal thereof after use.